Most optical systems are designed with consideration of the optical aberrations internal to the system only. Careful selection of optical surface geometry combined with precise fabrication, careful assembly, and inclusion of a select few adjustable parameters (e.g. focus, zoom, or spherical aberration correction) allow the optical system to achieve a specified nominal level of performance. However, if a source of optical aberration exists outside of the optical system and the aberrations are unknown and possibly changing with time, the performance of the optical system can be significantly degraded. A select few examples of beam scanning imaging systems and sources of aberration are shown in FIG. 1 and FIG. 2, respectively. Adaptive optics (AO) provides a means to reduce the wavefront distortions caused by the source of aberration to achieve improved performance. In most AO systems, a wavefront correcting device (often a deformable mirror or liquid crystal spatial light modulator) contains several to thousands of individually addressable actuators or cells (pixels) to affect the wavefront, as shown in FIG. 3. Undesirable distortions to the wavefront can be corrected or a more preferable wavefront shape generated with the wavefront correcting device integrated in the optical system. Adaptive optics has been applied to correct for dynamic atmospheric aberration for telescope viewing, to correct for aberrations in the human and animal eye for retinal imaging, to correct for sample induced aberrations for microscopic imaging, to correct for sample induced aberrations in laser material processing, to correct for atmospheric aberrations for line of sight optical communications, and other applications where wavefront correction is desirable. The benefits of adaptive optics are generally improved resolution and signal strength in viewing or imaging applications, tighter focus and higher power density in beam projection applications, or improved communication rates in data transmission applications.
A paper, “The Possibility of Compensating Astronomical Seeing”, H. W. Babcock, Publications of the Astronomical Society of the Pacific, Vol. 65, No. 386, p. 229 (1953) first introduced the adaptive optics concept for astronomical viewing with earth based telescopes. The vast majority of adaptive optics systems to date have used the basic AO framework proposed in Babcock's paper with the system containing a wavefront sensor 410, an adaptive optics element 420, and a feedback control system 430 that takes input from the wavefront sensor and generates control signals to drive the adaptive optics element to a preferred wavefront correction shape, as shown in FIG. 4(A). The wavefront sensor could be of a Shack-Hartmann, pyramid, or other wavefront sensing design. An alternate and more recent implementation of AO does not use a wavefront sensor, but instead uses information about the quality of the measured signal as obtained by the image sensor 440 as the input to an optimization algorithm running on an optimization system 450 as part of the process to generate wavefront corrections for the adaptive optics element 460 for improved performance, as shown in FIG. 4(B). Implementing AO in this manner when the wavefront correction is not known a priori and without a dedicated wavefront sensor is commonly referred to as sensorless AO. A third variation of AO uses stored or calculated control signals applied to the adaptive optics element 470 by an open loop control system 480, referred to as open loop AO, as shown in FIG. 4(C).
AO System Aberration Challenges Taught by AO-SLO Examples
The historical challenges of managing system aberrations are described in the context of adaptive optics scanning laser ophthalmoscopes (AO-SLO). It has been long known that the peripheral cornea and crystalline lens in the human eye introduce wavefront distortions that degrade resolution at large pupil diameters. A paper, “Optical quality of the human eye” by F. W. Campbell and R. W. Gubisch, Journal of Physiology, Vol. 186, no. 3, pp. 558-578 (1966), finds that a pupil diameter of 2.4 mm yields the highest optical resolution using linespread analysis. Similar findings in a more recent paper, “Optimal pupil size in the human eye for axial resolution” by W. J. Donnelly III and A. Roorda, JOSA. Vol. 20, Issue 11, pp. 2010-2015 (2003), indicate that a pupil size of 2.46 mm provides the best lateral resolution and 4.6 mm provides the best axial resolution for traditional (non-AO) scanning laser ophthalmoscope (SLO) imaging. The aberration associated with larger pupil sizes dominates and degrades resolution to a greater extent than the improvement of resolution expected with the increasing numerical aperture and associated improved diffraction limit. An adaptive optics element can correct the peripheral cornea and crystalline lens aberrations to allow larger pupil diameters to be used at or near the diffraction limit to achieve significantly improved resolution and imaging performance.
A paper, “Active optical depth resolution improvement of the laser tomographic scanner” by A. Dreher, J. Bille, and R. Weinreb, Appl. Opt. 28, 804-808 (1989) teaches using a deformable mirror in an open loop manner to correct for aberrations in the human eye at a pupil diameter of 6 mm to achieve a two-fold increase in depth resolution in a laser tomographic scanner. Further, the same paper teaches using an afocal 4f arrangement of lenses in a relay configuration to image the active surface of the deformable mirror to the entrance pupil of the eye. An additional afocal 4f arrangement of lenses images the scan pupil of a galvanometer (galvo) scanner to the active surface of the deformable mirror. This basic arrangement and use of multiple 4f relays between the eye, AO element, and scanners has become the standard for nearly all AO systems that perform laser scanning ophthalmic imaging, although the ordering of pupil planes and specific optical components used in the 4f relay can differ. If an additional galvanometer is used to perform 2D scanning, an associated additional 4f relay is used for proper pupil conjugation to the other scanner, adaptive optics element, and pupil planes. The design of the 4f pupil relay has been challenging because off-axis aberrations in the imaging system itself can introduce significant wavefront distortions. The problem is exacerbated because aberrations compound as multiple 4f relays are cascaded in series.
Early point scanning adaptive optics imaging systems used spherical mirrors in off-axis configurations to perform the 4f pupil relay and primarily concentrated on optimizing the image plane performance, as is described in a paper, “Adaptive optics scanning laser ophthalmoscopy” by A. Roorda, F. Romero-Borja, W. Donnelly, III, et al, A. Roorda, F. Romero-Borja, W. Donnelly, III Opt. Express 10, 405-412 (2002) and a related U.S. Pat. No. 6,890,076 B2. However, the in-plane configuration of pupil relays used in this paper and patent is known today to generate considerable residual astigmatism aberration which degrades imaging performance.
A paper “Large-field-of-view, modular, stabilized, adaptive-optics-based scanning laser ophthalmoscope” by S. Burns, R. Tumbar, A. Elsner et al, J. Opt. Soc. Am. A 24, pp. 1313-1326 (2007), teaches that even with small off-axis beam angles on the spherical mirrors in the 4f pupil relay, off-axis astigmatism accumulates with multiple sequential mirror reflections in the system. The paper teaches that designing the optics such that the second pupil relay is constructed out-of-the-plane compared with the first pupil relay, astigmatism can be partially cancelled. A paper, “First-order design of off-axis reflective ophthalmic adaptive optics systems using afocal telescopes” by A. Gómez-Vieyra, A. Dubra, D. Malacara-Hernindez, and D. Williams, Opt. Express 17, pp. 18906-18919 (2009), further investigates off-axis aberrations and develops associated theory to optimize imaging performance in the both the retinal (imaging) and pupil planes by also using an out-of-plane relay configuration. A follow up paper, “Geometric theory of wavefront aberrations in an off-axis spherical mirror” by A. Gomez-Vieyra and D. Malacara-Hernández, Appl. Opt. 50, pp. 66-73 (2011), extends the aberration theory of pupil relays to higher orders and is used as the basis for an improved ophthalmic AO imaging system described in a paper, “Reflective afocal broadband adaptive optics scanning ophthalmoscope” by A. Dubra and Y. Sulai, Biomed. Opt. Express 2, pp. 1757-1768 (2011).
Indeed, the importance of minimizing aberration, and particularly astigmatism, as well as simultaneously minimizing both the aberrations in the imaging planes and the pupil planes was demonstrated by two groups independently publishing images of the elusive rod mosaic in the papers, “Noninvasive imaging of the human rod photoreceptor mosaic using a confocal adaptive optics scanning ophthalmoscope” by A. Dubra, Y. Sulai, J. Norris, R. Cooper, A. Dubis, D. Williams, and J. Carroll, Biomed. Opt. Express 2, pp. 1864-1876 (2011) and “Observation of cone and rod photoreceptors in normal subjects and patients using a new generation adaptive optics scanning laser ophthalmoscope” by D. Merino, J. Duncan, P. Tiruveedhula, and A. Roorda Biomed. Opt. Express 2, pp. 2189-2201 (2011). This second paper also teaches that in addition to introducing scan position dependent wavefront aberrations in both the image and pupil planes, beam wandering also occurs in spherical mirror based 4f pupil relay systems. Beam wandering can be improved with the out-of-plane relay configuration.
Over the course of over a decade, AO based SLO imaging has advanced considerably from systems that could only resolve the relatively large peripheral cone mosaic to being able to resolve the very small rod mosaic in the retina. Paying close attention to the details of the aberrations and quality of pupil relay has been a major contributor to the ever improving imaging performance. However, the resulting size of these new optimized AO imaging systems is quite large due to the long focal lengths of the spherical mirror components used in the highly optimized designs. For example, in the previously mentioned optimized designs, the afocal telescope is over 1.5 meters in length (Dubra, 2011) and 0.4 meters in length (Merino, 2011) because long focal length mirrors are used to reduce off-axis aberration. The large size of the spherical mirror based AO systems is compounded by the need to cascade multiple afocal relays in the AO system, each of considerable length of its own.
Positively powered mirrors and reflective surfaces have been most commonly used in AO-SLO systems because the small back reflections from glass or lens surfaces are significant and can interfere with measurement of the small levels of light returning from the retina. Glass surface back reflections can also generate stray light artifacts and ghost images that degrade wavefront measurement with a wavefront sensor. For these reasons, mirrors have been preferred over lenses and have been used almost exclusively in high performance AO-SLO systems, as described in the before mentioned paper (Gómez-Vieyra, 2009).
A paper, “Lens based adaptive optics scanning laser ophthalmoscope” by F. Felberer, J. Kroisamer, C. Hitzenberger, and M. Pircher, Opt. Express 20, 17297-17310 (2012), teaches that an all lens based implementation of the multiple afocal pupil relays used in an AO-SLO system can achieve a comparable level of aberration as the more complicated out-of-plane spherical mirror based configuration. The lengths of the afocal pupil relays are on the order of 0.5 meters. The problem of backreflections from the glass surfaces interfering with the wavefront measurement is addressed by introducing a polarization beam splitter and polarizer in front of the wavefront sensor and a quarter waveplate in front of the eye such that light reflected from glass surfaces is rejected, but light reflected from the eye is passed through to the wavefront sensor. The problem of backreflections from lens and glass surfaces interfering with the image detection and formation is not addressed. The paper shows results of the rod mosaic, although the quality of the image does not look as good as the images obtained with the all mirror based out-of-plane configuration of the before mentioned Dubra 2011 paper.
The discussion so far has focused on AO-SLO because this technology is one of the most well documented and carefully analyzed of the adaptive optics systems. Other AO systems using different imaging modalities or material processing capability have also been demonstrated and have faced the same off-axis aberration and size challenges, as well as additional challenges associated with dispersions in glass elements when short pulsed lasers are used.
Microscope Imaging with Adaptive Optics
High performance microscope objectives achieve optimal performance when imaging under well controlled and prescribed imaging conditions. Small perturbations to nominal imaging conditions can result in a significant reduction of signal strength and a degradation of resolution. Detrimental perturbations to nominal imaging conditions can arise from using different thickness coverslips, using an oil immersion objective in a water immersion imaging scenario, from imaging into tissue or other samples, from imaging through sample containers, or from other sources. A paper, “Aberration correction for confocal imaging in refractive-index-mismatched media” by M. J. Booth, M. A. A. Neil, and T. Wilson, Journal of Microscopy, Vol. 192, issue 2, (1998) analyzes specimen and sample induced aberration and teaches the potential of using a deformable mirror in a confocal or two photon microscope to correct for aberrations occurring from deep imaging through refractive index mismatched media.
A paper, “Adaptive aberration correction in a two-photon microscope” by M. A. A. Neil, R. Juskaitis, M. J. Booth, T. Wilson, T. Tanaka, and S. Kawata, J. Microscopy, Vol. 200, Pt. 2, pp. 105-108 (2000), describes the first experimental application of two photon imaging with adaptive optics. The adaptive optics corrector, a ferroelectric liquid crystal spatial light modulator (FLCSLM), is located before the scanning mechanism in a commercial laser scanning microscope.
A patent, U.S. Pat. No. 6,381,074 B2, teaches a laser scanning microscope that includes a wavefront converting element to perform scanning of the focus in the optical axis (depth) direction without the need to change the distance between the microscope objective and the specimen. Aberration occurring during the depth scanning is canceled by using the wavefront converting element to minimize the degradation of light collecting performance due to the scanning in the optical axis direction. The wavefront converting element is placed at or near a position conjugate to the objective pupil position so that predetermined conditions are satisfied. Further, the wavefront converting element and each of two galvanometer mirrors in the scanning optical system to scan the position where light is collected in a direction perpendicular to the optical axis and further the pupil position of the objective are all placed in conjugate or nearly conjugate relation to each other by the intervening optical systems. The scanning optical system includes a pupil projection lens for placing the wavefront converting element and the galvanometer mirror closer to the wavefront converting element in conjugate relation to each other.
A paper, “Smart microscope: an adaptive optics learning system for aberration correction in multiphoton confocal microscopy” by O. Albert, L. Sherman, G. Mourou, T. Norris, and G. Vdovin, Opt. Lett. 25, 52-54 (2000) teaches using a deformable mirror to correct for off-axis aberrations in a two photon imaging system. The objective is an off-axis parabolic mirror and the intensity of a two photon sample is used to optimize the deformable mirror shape.
A paper, “Adaptive aberration correction in a confocal microscope” by M. J. Booth, M. A. A. Neil, R. Juskaitis and T. Wilson, Proc. Nat. Acad. Sci., Vol. 99, No. 9, 30, pp. 5788-5792 (2002), describes the first demonstration of adaptive optics in a confocal microscope. The paper teaches using relay lenses between the deformable mirror and the objective.
A paper, “Adaptive correction of depth-induced aberrations in multiphoton scanning microscopy using a deformable mirror” by Sherman L, Ye J Y, Albert O, Norris T B. J Microsc. 206 (Pt 1):65-71 (2002), demonstrates using a deformable mirror as the wavefront corrector in a multiphoton scanning microscope. The paper teaches using a 4f telescope system to directly image the face of the DM to the entrance pupil of the microscope objective.
A patent, U.S. Pat. No. 6,771,417 B1, teaches the use of one or more wavefront modulators in the observation beam path and/or illumination beam path of a microscope. The patent teaches placing the wavefront modulator between the tube lens and the objective. Such modulators may be adapted to change the phase and/or the amplitude of light in such a way to carry out displacement and shaping of the focus in the object space and correction of possible aberrations. An embodiment of the invention allows focusing to different depths without changing the distance from the objective to the object. The possible areas of use include confocal microscopy, laser-assisted microscopy, conventional light microscopy and analytic microscopy.
A patent, U.S. Pat. No. 7,733,564 B2 (continuation patent of above mentioned U.S. Pat. No. 6,771,417 B1), includes additional claims in which a design change to the instrument of placing the wavefront modulator in a pupil plane is claimed, although the methods and mechanisms for doing so are not described.
A patent, U.S. Pat. No. 7,659,993 B2, teaches a wavefront sensing device within an adaptive optics microscope architecture. An embodiment of the invention is described for fluorescent imaging with examples of multi-photon and confocal microscopy. A wavefront sensor uses interferometric techniques, called coherence gating, to isolate a depth of interest in the sample. The deformable mirror is adapted to a predetermined shape in order to form the desired wave-front of the travelling light pulses. Specimen scanning is obtained with movement of the specimen holding device.
The challenges of using the before mentioned and traditional approach of cascading multiple pupil relays in microscopy has been recognized. A patent, U.S. Pat. No. 7,002,736 B2, teaches a scanning optical microscope using a wavefront converting element to correct for aberrations. Citing Japanese patent, HEI-11-101942 4 (1999), which teaches that it is desirable that the wavefront converting element should be placed at a position conjugate to the pupil, the patent emphasizes that it is difficult to implement pupil relay systems because of the following problems. A first problem is that a variety of objectives are used in microscopic observation, and the pupil position differs for each objective. Therefore, when a plurality of objectives are switched from one to another to perform observation, it is difficult to keep the pupils of the objectives in conjugate relation to the wavefront converting element at all times. Further, the wavefront converting element needs to be placed in conjugate relation to the position of a laser scanning member and also to the position of the objective pupil. Accordingly, at least two pupil relay optical systems are required. Therefore, the apparatus becomes large in size and complicated unfavorably.
Adaptive optics have been used in a microscope for reasons other than to correct optical aberrations. A patent, U.S. Pat. No. 8,198,604 B2, teaches a system for providing enhanced background rejection in thick tissue that contains an aberrating element for introducing controllable extraneous spatial aberrations in an excitation beam path. An associated method comprises the steps of acquiring two-photon excited fluorescence of thick tissue without extraneous aberrations; introducing an extraneous aberration pattern in an excitation beam path; acquiring two-photon excited fluorescence of the thick tissue having the introduced extraneous aberration pattern; and subtracting the two-photon excited fluorescence with extraneous aberrations from the acquired standard two-photon excited fluorescence of the thick tissue without extraneous aberrations. The deformable mirror is relayed to the beam scanner, which is in turn relayed to the back aperture of the objective. The deformable mirror is located in a conjugate plane of the objective back aperture.
OCT Imaging with Adaptive Optics
Similar to AO-SLO, adaptive optics has been applied to Optical Coherence Tomography (OCT) for adaptive optics OCT (AO-OCT).
A patent, U.S. Pat. No. 7,364,296 B2, teaches a method of optical imaging comprising providing a sample to be imaged, measuring and correcting aberrations associated with the sample using adaptive optics, and imaging the sample by optical coherence tomography.
A patent, U.S. Pat. No. 7,942,527 B2, teaches using a Badal optometer and rotating cylinders inserted in an AO-OCT system to correct large spectacle aberrations such as myopia, hyperopic and astigmatism for ease of clinical use and reduction. Similar to as implemented with AO-SLO, spherical mirrors in the telescope are rotated orthogonally (out-of-plane) to reduce aberrations and beam displacement caused by the scanners. This produces greatly reduced AO registration errors and improved AO performance to enable high order aberration correction in patient eyes.
A patent, U.S. Pat. No. 7,896,496 B2, teaches an object tracking system that can be used for AO-SLO or AO-OCT.
A patent application, WO2005060823 A1, teaches a data acquisition system where measurements are made by OCT, wherein a quality of these measurements is improved by arranging an active optical element in the beam path, the system also including a wavefront sensor.
A patent application. US20120019780 A1, teaches an AO-SLO or AO-OCT.
A patent application, US20110234978 A1, teaches a multifunctional optical apparatus that includes a system of optical components capable of operating in a scanning laser ophthalmoscope (SLO) mode and an optical coherence tomography (OCT) mode. Multiple scanning devices are positioned at pupil conjugates in the system of optical components. The system may include optical tracking along with one or more optional adaptive optics.
A patent application, US20120002165 A1, teaches an invention that can image with SLO or OCT that has multiple measuring beams and uses adaptive optics that include: a wavefront aberration detector for detecting a wavefront aberration in a reflected or backscattered beams generated when a plurality of beams are scanned on a surface, and a single wavefront aberration corrector for correcting a wavefront aberration in each of the plurality of beams, based on the wavefront aberration, and the plurality of beams enter the single wavefront aberration corrector with different incident angles and are overlapped on each other. In one embodiment, the wavefront aberration corrector is disposed at a position at which an exit pupil of relay optics is acquired optically conjugate with the single position at which the plurality of beams intersect with each other.
A patent application, US20120044455 A1, teaches an AO-SLO or AO-OCT imaging apparatus using a deformable mirror and wavefront sensor. Pupil relay optics are used and the patent application teaches that relay lenses are used so that the cornea, the XY scanner, and the wavefront sensor become approximately optically conjugate with each other.
Material Processing and Object Manipulation with Adaptive Optics
Various papers have described using adaptive optics for beam shaping in material processing applications, including a paper, “Beam delivery by adaptive optics for material processing applications using high-power CO2 lasers” by Heinz Haferkamp and Dirk Seebaum, Proc. SPIE 2207, Laser Materials Processing: Industrial and Microelectronics Applications, 156 (1994), and a paper, M. Geiger, Synergy of Laser Material Processing and Metal Forming, CIRP Annals—Manufacturing Technology, Volume 43, Issue 2, pp. 563-570 (1994).
Adaptive optics have been used to correct for sample induced aberration in material processing. A paper, “Active Aberration Correction for the Writing of Three-Dimensional Optical Memory Devices” by M. Neil, R. Juskaitis, M. Booth, T. Wilson, T. Tanaka, and S. Kawata, Appl. Opt. 41, 1374-1379 (2002), teaches using an SLM to compensate for sample induced aberrations when writing 3D optical memory devices. A paper, “Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction”, C. Mauclair, A. Mermillod-Blondin, N. Huot, E. Audouard, and R. Stoian, Opt. Express 16, 5481-5492 (2008), teaches using an SLM in a laser processing system to improve the quality of laser processing. A paper, “Adaptive optics for direct laser writing with plasma emission aberration sensing” by A. Jesacher, G. Marshall, T. Wilson, and M. Booth, Opt. Express 18, 656-661 (2010), teaches using an SLM in a plasma emission direct laser writing system.
Adaptive optics have been used for optical manipulation. One method of manipulating small objects is to use optical trapping, sometimes referred to as optical tweezers. Most methods of using optical tweezers do not include a galvo based scanning mechanism as taught in the following papers: “Adaptive optics in an optical trapping system for enhanced lateral trap stiffness at depth”, by M C Miillenbroich, N McAlinden and A J Wright, M C Mfillenbroich et al, J. Opt. 15 075305 (2013), a paper, “Holographic optical tweezers aberration correction using adaptive optics without a wavefront sensor” by K D. Wulff, D G. Cole, R L. Clark, R D Leonardo, J Leach, J Cooper, G Gibson, M J. Padgett, Proc. SPIE 6326, Optical Trapping and Optical Micromanipulation III, 63262Y (2006), and a thesis, “Design and characterization of an optical tweezers system with adaptive optic control” by S. Bowman (2009).
More advanced optical trapping setups include scanning and/or beam splitting capability, such as a paper, “Combined holographic-mechanical optical tweezers: Construction, optimization, and calibration”, by Richard D. L. Hanes, Matthew C. Jenkins, and Stefan U. Egelhaaf, Rev. Sci. Instrum. 80, 083703 (2009). In this paper, the SLM is placed near the objective and not explicitly conjugated to the aperture. The SLM allows multiple traps to be formed such that the galvos can do coarse steering of the beam and the SLM can perform beam splitting to generate multiple traps and fine steering of the beam. The deformable mirror used in the apparatus is calibrated by optimizing oscillatory drag force on a trapped object.